1. Field of the Invention
This invention relates generally to a fuel cell system that employs an algorithm providing a strategy for maintaining the cathode outlet gas humidity and, more particularly, to a fuel cell system that employs an over-arching algorithm for maintaining the cathode outlet gas humidity at a wet or dry mode of operation so as to prevent membrane humidity cycling.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. Each bipolar plate includes an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Excessive stack temperatures may damage the membranes and other materials in the stack. Fuel cell systems therefore employ a thermal sub-system to control the temperature of the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling fluid flow channels in the bipolar plates in the stack to draw away stack waste heat. During normal fuel cell stack operation, the speed of the pump is controlled based on the stack load, the ambient temperature and other factors, so that the operating temperature of the stack is maintained at an optimal temperature, for example 80° C. A radiator is typically provided in a coolant loop outside of the stack that cools the cooling fluid heated by the stack where the cooled cooling fluid is cycled back through the stack.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH). The relative humidity of the cathode outlet gas from the fuel cell stack is controlled to control the relative humidity of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack. For stack durability purposes, it is desirable to minimize the number of relative humidity cycles of the membrane because cycling between RH extremes has been shown to severely limit membrane life. As used herein, RH cycling refers to a changing membrane relative humidity above and below 100%. Membrane RH cycling causes the membrane to expand and contract as a result of the absorption of water and subsequent drying. This expansion and contraction of the membrane causes pin holes in the membrane, which create hydrogen and oxygen cross-over through the membrane creating hot spots that further increase the size of the hole in the membrane, thus reducing its life.
During cold start-up of the fuel cell system, the relative humidity of the cathode outlet gas would typically exceed 100% because the cold cathode exhaust gas is unable to hold very much water vapor. As the fuel cell stack heats up, and the load on the stack increases, the heat rejection capability of the thermal sub-system would typically not be sufficient to sustain the cathode outlet gas relative humidity over 100% because this would require a large thermal sub-system cooling capacity. Therefore, a transition of reducing the relative humidity of the cathode outlet gas would occur to a dryer membrane condition. Many factors go into whether the fuel cell system is operating in a wet or dry condition, including load on the stack, ambient temperature, cooling capacity of the thermal sub-system, etc.
As the operating conditions of the fuel cell system change, the relative humidity of the cathode outlet gas tends to cycle back and forth between a wet operating condition and a dry operating condition. As used herein, the wet operating condition refers to a relative humidity of the cathode exhaust gas greater than 100% and the dry operating condition refers to a relative humidity of the cathode exhaust gas less than 100%. Therefore, it would be desirable to reduce the membrane RH cycling during operation of the stack to increase the life of the membranes in the stack.